Abstract: A positive displacement rotary pump for providing substantially pulse free fluid transfer including a pair of intermeshed cylindrical rotor members having alternating helically arranged ribs and flutes. The pitch and dimensions of the ribs and flutes is such that a rib-flute pair just breaks contact as the adjacent rib-flute makes contact, thereby presenting a substantially constant driving surface to the fluid resulting in continuous fluid flow through the pump. In a preferred embodiment, helical drive gears having teeth pitched oppositely to the rotor member ribs are positioned outside of the fluid flow path to drive one of the rotor members from the other.